The purpose of a voltage regulator is to provide a predetermined and substantially constant output voltage to a load from a voltage source which may be poorly-specified or fluctuating. Two types of voltage regulators are commonly used to provide this function, a linear regulator and a switching regulator. In a typical linear regulator, the output voltage is regulated by controlling the flow of current through a pass device from the voltage source to the load.
In switching voltage regulators, however, the flow of current from the voltage source to the load is not steady, but is rather in the form of discrete current pulses. To create the discrete current pulses, switching regulators usually employ a switch (such as a power transistor) that is coupled either in series or parallel with the load. The current pulses are then converted into a steady load current with an inductive storage element.
Current-mode control has been used in switching power regulators for many years. One of the primary advantages of current mode control is that the circuit goes gracefully into output or peak current limit. This output current limit is also known as the current trip threshold because the regulator will no longer source any more current and the output voltage will droop until the system reaches equilibrium when the peak current limit is reached. In this way the load to which the regulator may be connected is protected from current overload.
In all current mode controllers, the current limit is established as a maximum voltage allowed across a current sensing element (typically the current sensing element is the DCR of the inductive storage device, but also can be a separate resistance element). The user can adjust the maximum current allowed by modifying the current sensing element. In some controllers, the current limit can also be adjusted by modifying the transconductance of the error amplifier used in providing an error signal as a function of the input and output signals. Both of these methods modify the loop gain of the network and require the compensation to be modified.
For most current mode applications this is not a problem. The current limit is one of the parameters established during the design phase and the compensation network is optimized for all the system requirements including the current limit. Finally, these parameters are fixed for the application and do not require adjustment.
There are new applications for DC/DC converters have the ability to modify the output parameters of the system during operation. To assure proper operation of the circuit, it is desirable to have the current limit threshold modified without affecting the loop gain. This assures optimal stability without the requirement to modify the compensation loop.
Further, the resistance of the sensing element can vary with temperature, which is particularly true where the resistance of the sensing element is the DCR of the inductive storage element.